Combustible containers for propellant compositions in commercial use at the present time typically are fabricated by a felting process utilizing paper or cardboard materials. An illustrative container material is Kraft paper employing 13.4 percent nitrogen containing nitrocellulose, and the paper can be coated, impregnated or dipped to incorporate various optional additives as desired.
Among the disadvantages of the prior art combustible containers is the tendency to leave a burn residue in the combustion chamber of the gun, and the tendency to absorb moisture during long term storage. Accordingly, new containers which are clean burning during use and are moisture resistant are highly desired by the ammunition manufacturing community.
Conventional combustible cases are constructed by a felting process or spiral wrapping nitrocellulose impregnated Kraft paper on a mandrel and then soaking the case so formed in a solution such as a polyurethane and solvent. The polyurethane and solvent is distributed through the tubular body of the case. This soak yields a concentration gradient of polyurethane across the wall thickness of the tubular case. Conversely, the polyurethane resin may be premixed with the felting nitrocellulose and kraft fibers, eliminating the polyurethane soaking step. In addition, the voids left during the drying process prevents a perfect moisture seal. Thus the conventional case so constructed is not completely moistureproof.
In addition, the conventional case contributes minimal energy into the propulsion system. The propellant charge carried within the case in conventional combustible cartridge cases must have sufficient energy to generate the bulk of the propelling gases as well as incinerate the casing in order to leave no residue within the gun chamber.
Separately applied coatings help to provide a moisture seal. However, to provide this seal, the coating must be nonporous, which tends to inhibit complete combustion of the case. Accordingly, an energetic material can be incorporated into this coating.
One energetic protective coating for a caseless propellant charge is disclosed in U.S. Pat. No. 3,730,094. In this case a liquid mixture is applied to the exterior of the caseless molded charge. The liquid mixture is an explosive such as HMX or RDX and cellulose acetate in acetone. The mixture is applied to the exterior of the molded propelling charge and then the acetone evaporated. Cellulose acetate is added to improve the high temperature characteristics of the mixture.
Another example of a combustible case coating is found in U.S. Pat. No. 3,927,616. This patent discloses an oil and water resistant coating of RDX dispersed in a polymeric resin such as an alkyl methacrylate polymer.
Another protective coating for a combustible case is disclosed in U.S. Pat. No. 3,987,731. In this patent, the case, made of nitrocellulose (NC), reinforcing fibers and a resin binder is coated first with polyvinyl alcohol resin and then with a copolymer of vinylidine chloride and acrylonitrile. The resultant case is said to be resistant to moisture and oil.
Another example of a protective coating for a caseless propelling charge or combustible casings is disclosed in U.S. Pat. No. 4,363,273 to Luebben et al. This patent discloses a two step composite coating for a porous NC case permeated with a polyurethane which has a first undercoating containing metal particles bonded to the case and an outer coating of a polyurethane resin. This composite coating allegedly makes the cartridge resistant to ignition when exposed to contact with hot surfaces, a very desirable feature. The outer coating provides the moisture seal.
Other patents describing coatings or moisture resistant combustible cases include U.S. Pat. Nos. 3,397,637; 3,670,649; 3,703,868; 3,706,280; 3,727,512; 3,769,873; 3,770,563; 3,877,374; 4,649,827; and 4,709,636.
A new casing for molding directly to a solid molded or compacted propelling charge is disclosed in copending U.S. patent application Ser. No. 07/576,576, filed Aug. 31, 1990, now U.S. Pat. No. 5,069,133, and assigned to the Assignee of the present invention. This copending application is incorporated herein in its entirety by reference. This copending application discloses an encased propellant such as Ball Powder.RTM. propellant made by Olin Corporation. The solid self supporting propellant charge is encased in an overwrapping of an elastomeric coating composition free of any cellulosic compound, and preferably comprises a polyurethane. As such, the coating is very thin and is thus completely incinerated during the ballistic cycle. Thus an energetic constituant in the coating is not needed.
In situations where a loose propellant charge, such as Ball Powder.RTM. propellant or stick propellant is used, a separate container made of nitrocellulose impregnated Kraft paper is generally used. These containers may be advantageously overwrapped as described in the above patents or as in the above mentioned copending application to provide a moisture proof casing. However, use of such a casing does not significantly contribute to the combustion process within the gun chamber. These cases typically produce about 5% of the total propellant system energy.
Therefore there is still a need for a casing which provides adequate protection from the environment, is totally consumed during the combustion process and at the same time contributes substantial energy to the combustion process.